1. Field of the Invention
The present invention relates to an optical coherence reduction method for reducing the coherence of a light beam and its device, an illuminating method in which a light beam the coherence of which is reduced is used and its system.
2. Description of the Related Art
A laser beam is applied to a light source for an image display, a light source for illumination for a semiconductor exposure-system and others. In these examples, the characteristics of a laser beam or coherent light such as high intensity, directivity and monochromaticity are used as advantages, however, in the meantime, a problem peculiar to coherent light such as a speckle (speckle noise) occurs and prevents the application.
Next, the above problem of a speckle will be described.
Generally, as shown in FIG. 8, when a laser beam has a unimodal power spectrum, that is, if the degree of coherence of a single mode laser beam is lg (xcfx84) l, lg (xcfx84) l becomes the following expression by Fourier transform G (xcfx84) of the power spectrum and the degree of coherence is shown in FIG. 9. (As described above, a laser oscillated at a single frequency or a single wavelength is called a longitudinal single mode laser.)
xe2x80x83lg(xcfx84)l≈lG(xcfx84)l/lG(0)1
If the full width at a half maximum (the spectral band width) of the unimodal power spectrum shown in FIG. 8 is vs, the full width at a half maximum xcfx84s of the degree of coherence shown in FIG. 9 approximately becomes 1/vs though the above full width at a half maximum also depends upon the function of the power spectrum.
xe2x80x9cxcfx84sxe2x80x9d and xe2x80x9cLcxe2x80x9d (xe2x80x9cLc=cxcfx84s:cxe2x80x9d shows light velocity in a medium at which light is propagated) are respectively called coherence time xcfx84s and coherence length Lc. That is, for example, a coherent luminous flux emitted from a coherent light source is branched into two and one optical path length is made longer by length L than the other optical path length. At this time, if the optical path length difference L is equal or smaller to/than coherence length Lc, these branched two beams have a large degree of coherence and mutually interfere. In the meantime, if the optical path length difference L is equal or larger to/than coherence length Lc, the two branched beams are mutually incoherent and no interference is caused between these two beams.
It is conceivable that when such a laser beam source is used for a light source for illumination of an image display for example, contribution from an object surface, for example each point and each area of a screen is collected and an image is formed on an image surface, for example on a retina of an observer.
At this time, as it is natural that there are irregularities equal to or large than a wavelength in depth on the object surface (for example, the screen), luminous fluxes having complicated phase relationship are overlapped on the image surface and if their fluxes are mutually coherent, they interfere as a result and a complicated contrastive pattern is formed. This is a speckle (or speckle noise) and in the case of a display, an image is remarkably impaired.
Therefore, to reduce a speckle, it is important to make a coherent luminous flux emitted from a coherent light source mutually incoherent.
For a first method to reduce a speckle, it is conceivable that the spectral band width of a light source is sufficiently widened, that is, coherence length is sufficiently reduced. However, as this method sacrifices the original characteristic of a laser beam, high monochromaticity, it is generally not desirable.
A second method is a method of merging again after coherent light emitted from a light source provided with coherence length to some extent is branched into multiple luminous fluxes and optical path length difference larger than approximately coherence length is given between each other. Hereby, as each luminous flux becomes incoherent, the degree of spatial coherence of merged laser beams can be reduced as the branch of luminous fluxes is increased.
For a device for realizing the second method, bundled fibers or an optical fiber bundle are/is known. In the bundled fibers, plural optical fibers are bundled and optical path length difference longer than the coherence length of an incident light beam is applied to the length of each optical fiber. Both ends of an optical fiber bundle are respectively arranged and a laser beam is incident from one end. Then, at the other end, outgoing laser beams from the respective optical fibers mutually become incoherent and the whole spatial coherence is decreased. Therefore, if such a beam is used for a light source for illumination, a speckle can be reduced.
However, to reduce a speckle of a single mode laser beam emitted from a laser beam source (a longitudinal single mode laser beam source) which oscillates a unimodal power spectrum shown in FIG. 8 using bundled fibers (an optical fiber bundle), there are some problems.
For example, if a semiconductor laser which oscillates in a longitudinal single mode is used for a light source, its typical spectral band width vs is approximately 100 MHz and therefore, its coherence length Lc is approximately 3 m. If 31 optical fibers are bundled and optical path length difference between each fiber is 3 m, difference in length between the shortest optical fiber and the longest optical fiber is 90 m.
Also, for example, as spectral band width vs is approximately 1 MHz if a light source is a helium-neon laser which oscillates in a longitudinal single mode, coherence length Lc is approximately 300 m and it is not realistic to generate such long optical path difference even if an optical fiber is used. That is, actually, the volume of a device is large as a whole.
As described above, to sufficiently reduce the degree of coherence of a laser beam in a single mode, after a laser beam in a single mode is branched into plural, optical path length difference exceeding coherence length is required to be applied every laser beam and heretofore, a small-sized and light optical device which can afford such optical path length difference is not obtained.
The object of the present invention is to provide an optical coherence reduction method and its device wherein coherence can be sufficiently reduced without applying optical path length difference exceeding coherence length to a branched light beam when the coherence of a coherent light beam is reduced.
Further, another object of the present invention is to provide an illuminating method and its system for utilizing a light beam the coherence of which is sufficiently reduced according to the above optical coherence reduction method for a light beam for illumination.
That is, the present invention relates to an optical coherence reduction method (hereinafter, called a coherence reduction method according to the present invention) wherein plural light beams are condensed in a single luminous flux or collected luminous fluxes after a light beam oscillated at plural different wavelengths is branched into plural and fixed optical path length difference is applied between the branched plural light beams.
According to a coherence reduction method according to the present invention, as plural light beams are condensed in a single luminous flux or collected luminous fluxes (that is, utilized as a luminous flux from a single light source as a whole) after a light beam (for example, a longitudinal multimode laser beam) oscillated at plural different frequencies (or wavelengths) is branched into plural and fixed optical path length difference is applied between the branched plural light beams, coherence can be sufficiently reduced without applying optical path length difference exceeding coherence length to the branched light beam.
The degree of coherence of a light beam oscillated at plural different frequencies periodically has a maximum value as the function of distance in case the oscillated frequency has periodicity. Therefore, if a light beam (a luminous flux) oscillated at plural different frequencies is branched into plural and fixed optical path length difference is applied to the branched each luminous flux to be a single luminous flux or collected luminous fluxes, spatial coherence can be sufficiently reduced even if the difference does not necessarily exceed coherence length unless the above fixed optical path length difference is equal to the maximum value of the degree of coherence.
Also, the present invention provides an optical coherence reduction device (hereinafter, called a coherence reduction device according to the present invention) wherein after a light beam oscillated at plural different wavelengths is branched into plural and fixed optical path length difference is applied between the branched plural light beams, plural light beams are condensed in a single luminous flux or collected luminous fluxes as a device for executing the coherence reduction method according to the present invention reproducibly.
Further, the present invention relates to an illuminating method (hereinafter, called an illuminating method according to the present invention) wherein after a light beam outgoing from a coherent light source which oscillates at plural different wavelengths is branched into plural and fixed optical path length difference is applied between the branched plural light beams the above plural light beams are condensed in a single luminous flux or collected luminous fluxes and the luminous flux is utilized as a light beam for illumination.
According to the illuminating method according to the present invention, as a coherent light source (for example, a longitudinal multimode laser beam source) which oscillates at plural different periodic frequencies is used for a light source, the degree of coherence of light beams having plural different frequencies outgoing from the light source periodically has a maximum value as the function of distance. Therefore, if a light beam (a luminous flux) oscillated at plural different frequencies outgoing from the coherent light source is branched into plural and fixed optical path length difference is applied to the branched each luminous flux to be a single luminous flux or collected luminous fluxes, spatial coherence can be reduced even if the above fixed optical path length difference does not necessarily exceed coherence length unless the above difference is equal to the maximum value of the degree of coherence.
Also, the present invention provides a luminaire (hereinafter, called a luminaire according to the present invention) provided with a coherent light source which oscillates at plural different wavelengths and optical means for branching a light beam outgoing from the coherent light source into plural and applying fixed optical path length difference between the branched plural light beams wherein the above plural light beams are condensed in a single luminous flux or collected luminous flux and utilized for light beams for illumination as a system for executing the illuminating method according to the present invention reproducibly.